A duplexer serves to separate transmitter and receiver signals in an FDD (Frequency Diversity Duplex) system and is used as a passive crossover network in the front end of a terminal device that serves as a transmitter and receiver. In the duplexer, the two bandpass filters can be interconnected a number of different methods in such a manner that simultaneous transmission and reception is possible. The objective in the development of duplexers is to minimize crosstalk. To this end, the transmitter and receiver paths must be extremely well insulated from each other.
With the increasing miniaturization and ever greater complexity due to multiband applications, duplexers for mobile terminal devices are integrated on modules. Because of miniaturization, the general problem is that such a module allows the mass of the duplexer to be connected to ground only to a limited extent since only a finite and therefore limited number of feed-throughs can be fitted on the module because its surface is limited.
A duplexer can be designed in the form of a discrete component with a configuration of two RF components as bandpass filters on a shared carrier substrate. This type of duplexer with a substrate and a chip disposed on said substrate and comprising a transmitter filter and a receiver filter is disclosed in U.S. Pat. No. 7,053,731 B2. Each of these filters comprises a ladder-type configuration of electro-acoustic resonators. However, duplexers can also have single filters implemented with other filter techniques or single filters that utilize different filter techniques.
As known from the above-mentioned U.S. patent, an inadequate ground connection causes a marked reduction of the transmitter/receiver insulation since current flowing to the ground generates a voltage drop across the inductance of the ground connection, which voltage drop affects all signal paths connected to this ground if the ground connection is inadequate. This voltage drop across the inductance is added vectorially to the basic insulation, which is determined by how the duplexer is otherwise wired and by the structure of the package.
When the connection of the component to ground is inadequate, properties, such as the selection of the component, can also be broadbandedly impaired in a single RF component, e.g., a filter.